A conventional multi-layer ceramic capacitor is typically formed as follows. A ceramic powder is dispersed in a polymeric material and the resulting slurry of the ceramic in the polymeric material with solvent is cast into a film form. This film is known as ceramic tape and may be cast anywhere from 0.5 mil up to 25 mils and even higher thicknesses. The ceramic tape has almost all of its solvent evaporated off before processing.
Metal electrodes are provided in a paste form. The metal particles are dispersed in a plasticized polymeric material with solvent to form a silk-screenable paste. The silk-screenable paste may contain from 40% to 80% metal powder by weight. This electrode paste is screened on the ceramic tape and dried. The tape itself may be diced or cut into individual tape pieces which are then stacked in such a way that individual capacitors are in parallel and the values add to each other.
The stack is then pressed to make a monolithic structure. Each individual unit is called a ceramic chip. The green chip is slowly heated to about 260.degree. C to burn off the plastic and then the ceramic is sintered at an appropriate temperature to achieve a fired monolithic multi-layer ceramic capacitor.
Alternate electrodes silk-screened onto the initial tape pieces forming the monolithic multi-layer ceramic capacitor are exposed at the two ends of the resulting chip and are separated by the ceramic dielectric. These electrode ends are connected to outside leads by fired on termination materials such as silver or palladium-silver. Leads are then normally soldered to the termination to make proper electrical connection.
Most conventional ceramic compositions or multi-layer ceramic capacitors contain barium titanate as a major starting chemical component. Small additions of compounds such as calcium zirconate, strontium titanate, bismuth titanate, calcium stannate, bismuth stannate, bismuth zirconate, lead titanate, barium zirconate, and the like change the electrical properties of barium titanate based ceramic compositions. For example, an 8-10% by weight addition of calcium zirconate to barium titanate shifts the Curie peak to room temperature in such a way that a temperature coefficient of capacitance can be obtained of from +20% to -56% over a temperature range of from 10.degree. C to 85.degree. C with 25.degree. C as a reference temperature.
Many other oxide compounds such as manganese oxide, silicon oxide, aluminum oxide, zirconium oxide, titanium dioxide, cobalt oxide, nickel oxide and the like are added to the ceramic composition in order to improve the sintering and electrical properties of the ceramic bodies. These ceramics have high dielectric constants up to 11,000 to 12,000. The high dielectric constant of the ceramic makes it attractive material for capacitor application.
Conventional ceramic compositions of the foregoing type for multi-layer ceramic capacitors are normally fired or sintered in air in the range of from 1200.degree. C to 1400.degree. C in forming the monolithic chip. These temperatures and air atmosphere require that the electrodes silk-screened onto the ceramic tape pieces comprise precious metals such as alloys of platinum, palladium and gold. In this respect, it is essential that the metal forming the electrode remain in conducting form while buried in the ceramic during the firing or sintering process. It is also essential that the metal electrode not adversely react with the ceramic and degrade the electrical properties of the ceramic. To maintain the electrode material conductivity it has to remain continuous during the firing or sintering of the ceramic. Any melting of the electrode during the sintering process results in discontinuities so that it will not have the necessary conducting properties for multi-layer ceramic capacitors.
While conventional precious metal electrodes such as platinum, palladium and gold and alloys thereof meet the necessary characteristics of continuity and inertness with respect to the ceramic material, they are relatively expensive and increase materially the overall cost of manufacture of multi-layer capacitors.
Typical prior art examples of ceramic capacitor material are found in U.S. Pat. Nos. 2,402,518; 2,443,211; 3,529,978; and 3,612,963.